A liquid crystal display device is a passive display device which mainly realizes picture display by changing the quantity of transmitted light of a backlight by means of deflection of liquid crystal molecules. Accordingly, the liquid crystal display device generally includes two parts, namely a liquid crystal panel and a backlight. The backlight generally has two types of light source: a cold cathode fluorescence lamp (CCFL) and a light emitting diode (LED). Compared with the CCFL, the LED has advantages of small size, long service life, high efficiency and the like. Therefore, nearly all the liquid crystal display devices available on the present markets are configured with LED backlights. FIG. 1 is a schematic diagram of a circuit composition of a LED backlight of a liquid crystal display device in the prior art. The circuit mainly includes a backlight driving module 110 and a LED lamp bead group 120, wherein the backlight driving module 110 provides a voltage signal and a current signal required by the LED lamp bead group 120 to emit light. The LED lamp beads are typically driven by direct current, and the brightness of emitted light of the LED lamp beads depends on the magnitude of the driving current. The magnitude of the driving current is generally adjusted according to a pulse modulation signal PWM input from outside.
At present, for a large-sized liquid crystal display panel, since the display area of the panel is relatively large and thus relatively high backlight brightness is needed, the light emitting time of the LED lamp beads must be prolonged. In other words, as higher driving current is needed for driving the LED lamp beads, the heat generated thereby is also increased, and the ambient temperature is thus raised. Under such case, the operation property of a LED lamp bead would be correspondingly changed with extension of the operation time, so that its service life is shortened and the light emitting quality is declined. Particularly, the color gamut is also changed, which affects the colors of pictures output by the liquid crystal display device.